The present invention relates to digitizer systems and, in particular, it concerns processing algorithms for ultrasound time-of-flight digitizer systems and systems employing such algorithms.
It is known to employ ultrasound time-of-flight measurements to measure distances in a two- or three-dimensional digitizer system. Such systems employ one transmitter which transmits ultrasound pulses which are received by receivers at a number of positions. Alternative configurations employ multiple transmitters with a single receiver. In either case, by measuring the time-of-flight of the pulses from the transmitter to the receiver, the distance between them can be calculated. Synchronization of the transmitter and receiver may be achieved either by a hard wired connection or by a wireless electromagnetic link.
Identification of the beginning of each ultrasound pulse received is non trivial. The reaction characteristic of transducer elements generally used to generate the pulses, together with dispersion of the signal during transit, cause the pulse to rise over a number of cycles before reaching its maximum amplitude. As a result, a threshold level set to reliably distinguish the pulse signals over background noise may be triggered at different stages of the pulse. Depending on the transmitter-receiver distance and various environmental conditions, the threshold may be exceeded sometimes during the first cycle, sometimes during the second, and sometimes during the third, leading to considerable imprecision (see FIGS. 1 and 2). For a typical operating frequency of about 40 kHz and taking the speed of sound to be 330 m/s, each cycle corresponds to a distance of about 8 mm. Such a range of error is unacceptable for typical applications such as digitizers for writing implements, computer mice and the like.
Conventional ultrasound digitizer systems are also somewhat inflexible in their hardware configurations. Typically, a predefined receiver arrangement is produced for each given application. The electronic components are then centralized in a control box. Such arrangements leave little or no flexibility to adapt systems to applications with larger dimensions.
There is therefore a need for systems and methods for accurately tracking variations in distance calculated from time-of-flight measurements of a sequence of pulses of a pressure wave from a transmitter to a receiver. It would also be highly advantageous to provide a modular receiver system in which the number of receiver units may be increased to cover any desired area.